Do police need to have established probable cause before they arrest you, or can your actions later validate the lack of probable cause that they had when they arrested you?

I suspect this may just be one of the many reasons not to talk to the police.

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    The police can do just about anything they want at that moment. Just don't be surprised if they actions are overturned on appeal, or result in a massive lawsuit against the jurisdition.
    – RonJohn
    Aug 25, 2021 at 18:34
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    Yes, the police can do whatever, but no one is really asking about or interested in what actions are theoretically possible for a human to perform. It's Law.SE, so everyone pretty well understands that we're talking about what the law permits.
    – bdb484
    Aug 25, 2021 at 19:41
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    Wouldn't the fact that the arrest was illegal make anything they learned based on that fruit of the poisonous tree? Aug 25, 2021 at 21:08
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    The usual approach is to arrest you for resisting arrest, then tack on an actual charge to justify the original arrest you resisted in the first place.
    – PcMan
    Aug 26, 2021 at 4:46
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    And yes, it is legal to arrest a person for "resisting arrest" and no other charge at all. It is highly unusual, but is allowed, practiced, and people are sitting in jail right now for just that.
    – PcMan
    Aug 26, 2021 at 4:55

5 Answers 5


Police may not arrest you without probable cause, and the existence of probable cause is evaluated “at the moment of the arrest.” Beck v. State of Ohio, 379 U.S. 89, 96 (1964). Therefore, police may not “look for after-the-fact justifications for [seizures] that would otherwise be impermissible.” United States v. Hughes, 606 F.3d 311, 316 (6th Cir. 2010).

In Beck, for instance, the defendant was arrested because he had a prior record of illegal gambling and because an officer had heard "reports" about him doing something that was never specified. When the officer saw the defendant driving, he stopped him and searched his car, but found nothing. He arrested him anyway, and when they got to the jail, they found betting slips in his pocket. He was convicted under the local gambling ordinance based on that evidence, but the Supreme Court reversed the conviction:

The constitutional validity of the search in this case, then, must depend upon the constitutional validity of the petitioner's arrest. Whether that arrest was constitutionally valid depends in turn upon whether, at the moment the arrest was made, the officers had probable cause to make it — whether at that moment the facts and circumstances within their knowledge and of which they had reasonably trustworthy information were sufficient to warrant a prudent man in believing that the petitioner had committed or was committing an offense.

However, if they want to arrest you for Crime A but only have probable cause to arrest you for Crime B, they may arrest you on that offense, and doing so may quite easily give them the time or access they need to gather additional evidence on Crime A. This is why so many large drug busts start as turn-signal violations.

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    Good answer. Regarding the last sentence, though, that's not quite the same situation, is it? Turn signal violations generally aren't criminal or arrestable offences, are they? Of course, you can be pulled over and written a ticket for it and anything the officer might happen to see in your car in the process is fair game for evidence of a different offence.
    – reirab
    Aug 25, 2021 at 16:52
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    @reirab You can be arrested for a traffic violation, yes.
    – D M
    Aug 25, 2021 at 22:21
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    It's going to vary from state to state, and with the level of the offense. You generally can't be arrested for speeding in Ohio, but drunk driving is obviously a different situation.
    – bdb484
    Aug 26, 2021 at 1:55
  • spped at 75 vs 150 ? One is a ticket. the other an arrest for charges related to reckless driving and endangerment. IANAL Aug 26, 2021 at 12:05

It depends on the meaning of "can." Do you mean to ask whether it is lawful, or whether it is possible for police to make such an arrest, or whether it is possible for such an arrest to lead to a successful prosecution?

As Rock Ape correctly points out in another answer, such an arrest is unlawful. Nonetheless, police make arrests without probable cause all the time. In the most egregious cases, the prosecutor recognizes this and orders the release of the arrested person.

But the police don't have to prove that they had probable cause until later. If they discover something after the arrest that allows them to make a credible claim that they had probable cause at the time of the arrest, then they'll get away with it. The less credible their claim, the more likely it is for the arrest to be found unlawful, whether by the prosecutor or, later still, by a judge.

  • Shouldn't the later evidence be "fruit of the poisoned tree"?
    – Barmar
    Aug 25, 2021 at 15:01
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    @Barmar yes, it should. But if an officer claims falsely that the evidence was known at the time of the arrest, and if the court finds the claim credible (that is, if the court does not recognize that the claim is false), then the evidence won't be fruit of the poisoned tree as far as the court is concerned. And "as far as the court is concerned" is all that matters for the purpose of court proceedings.
    – phoog
    Aug 25, 2021 at 15:15
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    Of course, many injustices can occur when the police are dishonest. Unfortunately, they get away with it quite a bit.
    – Barmar
    Aug 25, 2021 at 15:18
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    Although I think in Law.SE one would generally interpret "can" as "legally permitted to". If someone asks "Can I drive with a blood alcohol level of 0.10?", they're obviously asking whether it's legal, not whether they'll always be caught and arrested. OTOH, it's certainly reasonable to say "No, but they can often get away with it"
    – Barmar
    Aug 25, 2021 at 15:26
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    @Barmar sure, but lots of people on YouTube seem to think that quoting legal requirements at police trying to arrest them will somehow make the police see the light and stop trying to arrest them. It just seemed like a point worth making. People sometimes seem to lose sight of the fact that government officers charged with administering justice sometimes skirt the law just as anyone does.
    – phoog
    Aug 25, 2021 at 16:10

Can police arrest you to later fish for probable cause?

Answer for (although presumably there are similar conditions that must be met in the ):

This would be an unlawful arrest, potentially aggravated by false imprisonment (and/or kidnapping).

The American phrase "probable cause" is not used in the UK but it (somewhat) equates to the two elements that make a lawful arrest required by s.24 of the Police and Criminal Evidence Act 1984:

  • Suspicion of an offence, and

  • Belief an arrest is necessary (if not met, then the officer has other options such as giving words of advice for low-level offending, issuing a Fixed Penalty Notice or Traffic Offence Report or releasing the suspect on "street bail" ​to appear at police station at a later date)

The arresting officer must reasonably satisfy both elements in his own mind (e.g. not just blindly following orders, or by applying unjustified suspect-profiling) at the time and not later with hindsight or serendipity - especially as the officer must take the suspect to a police station as soon as practicable to justify the arrest to another officer, independent of the investigation, who will or will not authorise the suspect's further detention. This gives little time to come up with a plausible story (although it does happen).

*(On a scale of 0 to 10 - with 10 being total knowledge and 0 being no opinion at all - suspicion may be as low as 2 or 3 whereas belief starts at 7 or 8.)

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    So, suppose Scotland Yard has evidence against Professor Moriarty, and want to ensure that any of their officers can arrest him on sight. Do they have to hold an all-staff seminar where they present the evidence against Moriarty, so that when Constable Wiggins of the Dockyards Patrol spots him, he can believe for himself that an arrest is necessary? Or is this handled some other way, e.g. Constable Wiggins merely detains Moriarty in some fashion until Inspector Lestrade, who's in on the investigation and does know the evidence, can show up to make the actual arrest? Aug 24, 2021 at 13:30
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    Okay, for suspicion then. How does Constable Wiggins form reasonable grounds for suspecting Professor Moriarty unless the case against Moriarty has been explained to him in detail? And I guess the same goes for necessity - how does Wiggins form reasonable grounds to believe that Moriarty is necessary, say, "to allow the prompt and effective investigation", unless he knows the specifics of the case? Aug 24, 2021 at 13:42
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    @NateEldredge Although seminars (rather Operational Briefings) are used very often for localised and/or targeted activity, there should also be enough detail of the allegations against the Prof. (or details of any warrant for his arrest) recorded on the Police National Computer (PNC) from which an officer can formulate the requirements to make an arrest (if the officer checks PNC that is) We call this "being circulated"
    – user35069
    Aug 24, 2021 at 14:21
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    @NateEldredge It seems like the obvious answer is that the officer who suspects Moriarty of something would go to the court and seek an arrest warrant, permitting any officer to make the arrest, regardless of his knowledge of the case. This is how we would do it in the U.S., anyway. Am I missing something?
    – bdb484
    Aug 24, 2021 at 15:00
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    @bdb484 I don't think you've missed anything, it's just that in E&W arrest warrants are not as prevelant as they seem to be in the USA.
    – user35069
    Aug 24, 2021 at 16:58

To further Rock Ape's answer for England and Wales police forces in the UK.

An offence can be dealt with in a number of ways, including the Street Bail or Voluntary Attendance (VA). This is when you take the suspect's contact details, let them go and will arrange for them to attend a police station to be interviewed for the offence at a later date.

So when is it okay to detain someone and remove their Human Right of Free Movement? To do this you must present at at least one of the following necessities of arrest (with a handy acronym of COPPLANID) at custody before they are put in a cell.

  • Child or Vulnerable Adult at risk - Think spouse beaters. Are they going to go back and further attack the victim?
  • Obstruction - Prevent unlawful obstruction of highway. See the latest Extinction Rebellion for example of that.
  • Public Decency - You've caught someone with their pants down; even if they do them up, do you think they'll just drop them again in 5 minutes.
  • Prevent Injury to self or others - Self in this case is the suspect. They are a suspected child molester and the lynch mob is outside.
  • Loss or Damage to Property - Someone has shopped them in; they let slip "I'm going key their car the f*****g snitch".
  • Address can't be ascertained. - You've got someone where the offence is normally dealt with by a ticket through the post. Except they refuse to give the address or the Officer believes the address they are giving is fake.
  • Name can't be ascertained - See above.
  • Investigation - Prompt and Effective Investigation - Are they are a drug dealer and you need to detain them before they get home and trash the evidence? Are they going to go off and intimidate a witness? You need to show that by letting them go and dealing with it at a later date, the investigation will be compromised.
  • Disappearance of person in question - The person has a history of leaving town, failure to attend VA, or court. You can detain this person and then they could be up in court the next day.

Once you have the necessity does it stay forever? No.

There are no magic words or special procedure for de-arresting someone. Say you arrest a spouse beater because you don't them going back again (Vulnerable Adult at risk), but after the initial incident has died down and the beater is in the back of the police car, the spouse packs a bag and says she is going somewhere that the offender won't know. Someone could argue that the necessity has gone as the offender no longer poses a risk to that vulnerable adult. Lawyer could argue that and get the continued detention classed as unlawful.

So to lawfully arrest someone you must have:

  1. An offence that you have witnessed or a strong suspicion that one has occurred (that will stand up to initial scrutiny in custody).
  2. A necessity that requires the arrest and further detention of a suspect.
  3. That necessity, to the officer's knowledge, still exists when arriving at custody.

In formal English common law, a person cannot be arrested unless they are in commission of a felony and the person or persons arresting them have observed them committing that felony. In all other cases, by formal law, a warrant of arrest would be needed. Note that there is no need to "arrest" someone to charge and prosecute them for a crime and the expectation under English common law is that a person is only arrested if there is need to do so, such as to identify the person. Otherwise, it is expected that if a person is thought to commit a crime, then what should happen is that they should be indicted, and then summoned to court where they face trial. None of this requires that the person be "arrested", only that it be known who they are, so that they can be summoned.

Obviously, in the modern world the police and the courts ignore this part of common law principles and arrest people on hearsay evidence after the fact, even for misdemeanors. One reason for this is that most states completely immunize police against being sued for false arrest. For example, in New Hampshire there is the following language:

Any claim arising out of an intentional tort, including assault, battery, false imprisonment, false arrest, intentional mental distress, malicious prosecution, malicious abuse of process, libel, slander, misrepresentation, deceit, invasion of privacy, interference with advantageous relations, or interference with contractual relations, provided that the employee whose conduct gives rise to the claim reasonably believes, at the time of the acts or omissions complained of, that his conduct was lawful, and provided further that the acts complained of were within the scope of official duties of the employee for the state.

So, in other words a policeman only has to show that he "reasonably believes" his conduct was lawful to be immune from a lawsuit for false arrest. Furthermore, I believe that New Hampshire's laws against false arrest (that criminalized false arrest) were deleted about 10 years ago.

However, there is a risk for the police in that if they were to engage in false arrest routinely, then there would be a political backlash and the laws against false arrest could be re-instated. For this reason police are cautious about arresting someone unless they have some kind of "reasonable" evidence that the person has committed at least a misdemeanor.

In New Hamsphire, if a person is arrested falsely, the usual course of action is to sue the police for false imprisonment, which is the only law remaining on the books for this kind of action. In some cases, in recent years courts have allowed these civil lawsuits to proceed if the police conduct was particularly egregious. For example, in one case a man was arrested and imprisoned for a long period of time when his drug addict girlfriend trained her 6-year-old daughter to say he raped her. He sued the police for false arrest after his lawyers finally forced the prosecutors to show the girl's record which featured a long list of similar lies against other people. The trial court dismissed the lawsuit, but on appeal the Supreme Court of NH vacated the trial court's ruling and allowed the lawsuit to proceed.

  • "and the expectation under English common law is that a person is only arrested if there is need to do so, such as to identify the person". This has now changed from a common law practice to Section G of the Police and Criminal Evidence Act 1984 Oct 6, 2021 at 14:30

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